DOI 10.1393/ncc/i2017-17067-8
Colloquia: IFAE 2016
IL NUOVO CIMENTO 40 C (2017) 67
Search for new supernova remnant shells in the Galactic Plane
with H.E.S.S.
M. Capasso(1)(∗), F. Brun(2), R. C. G. Chaves(3), C. Deil(4), A. Djannati-Ata¨ı(5), A. Donath(4), P. Eger(4), D. Gottschall(1), H. Laffon(6), V. Marandon(4), N. Maxted(3), L. Oakes(7), G. P¨uhlhofer(1), M. Renaud(3), A. Santangelo(1), M. Sasaki(1), R. Terrier(5), J. Vink(8) on behalf of the H.E.S.S. Collaboration
(1) Institut f¨ur Astronomie und Astrophysik, Abteilung Hochenergieastrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universit¨at - Sand 1, D 72076 T¨ubingen, Germany
(2) DSM/Irfu, CEA Saclay - F-91191 Gif-Sur-Yvette Cedex, France
(3) Laboratoire Univers et Particules de Montpellier, Universit´e Montpellier 2, CNRS/IN2P3, CC72 - Place Eugene Bataillon, F-34095 Montpellier Cedex 5, France
(4) Max-Planck-Institut f¨ur Kernphysik - P.O. Box 103980, D 69029 Heidelberg, Germany (5) APC, AstroParticule et Cosmologie, Universit´e Paris Diderot, CNRS/IN2P3, CEA/Irfu,
Observatoire de Paris, Sorbonne Paris Cit´e - 10, rue Alice Domon et L´eonie Duquet, 75205 Paris Cedex 13, France
(6) Universit´e Bordeaux, CNRS IN2P3, Centre d’ ´Etudes Nucl´eaires de Bordeaux Gradignan -33175 Gradignan, France
(7) Institut f¨ur Physik, Humboldt-Universit¨at zu Berlin - Newtonstr. 15, D12489 Berlin,
Germany
(8) GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam Science Park 904, 1098 XH Amsterdam, The Netherlands
received 17 October 2016
Summary. — It has been a long-standing question whether supernova remnants (SNRs) can account for the acceleration of the bulk of galactic cosmic rays up to the knee in the cosmic ray spectrum. TeV-selected SNRs are interesting sources to probe this hypothesis and the entire H.E.S.S. phase I data set of the Galactic Plane, collected over the past decade, constitutes a unique archive to scan. The idea of the presented work is to systematically search for new SNR candidates in the sky maps produced for the latest H.E.S.S. Galactic Plane Survey (HGPS), on the basis of their morphological shell-like appearance in the TeV band.
(∗) E-mail: capasso@astro.uni-tuebingen.de
2 M. CAPASSOet al.
Fig. 1. – Left: TeV surface brightness map of HESS J1534-571, with 3,4,5,6 σ significance con-tours overlaid; null-hypothesis H0 probability (Gaussian morphology) p = 6.4× 10−3. The
green contour indicates the outer boundary of the radio SNR candidate G323.7-1.0, detected in the 2nd MOLONGLO Galactic Plane Survey [1]. Given the presence of a matching radio counterpart the source was successfully identified as a supernova remnant. Right: TeV surface brightness map of HESS J1912+101, with 3,4,5,6,7 σ significance contours overlaid; H0
proba-bility p = 1.7× 10−6. This source currently lacks detected non-thermal emission in lower energy
bands and has therefore been classified as SNR candidate; for the time being it is likely the first TeV SNR without known counterparts in other wavebands.
1. – Introduction
Between 2004 and 2013 H.E.S.S. I(1) carried out a survey program that covered the
Galactic Plane at longitudes 65◦< l < 250◦and latitudes|b| 3.5◦. Survey and pointed observations in the survey regions add up to around 3000 hours of data after quality selection, in the range 0.1 E 100 TeV [2]. Depending on the analysis configuration, the point spread function for reconstructed γ-rays of the H.E.S.S. I array ranges from ∼ 0.05◦ to 0.1◦; together with a large field-of-view with ∅ 3◦ flat γ-ray acceptance,
this makes the instrument particularly suited for the discovery and study of extended TeV γ-ray sources in our Galaxy with typical sizes 0.5◦ [3].
2. – Search for TeV-SNR candidates: methodology
In order to search for new SNR candidates on the HGPS maps, a two-step approach was followed [4]: first, the survey maps were scanned on a 0.02◦× 0.02◦ grid and, at each grid point, a shell model (projected 3D shell, homogeneously emitting between Rin
and Rout) was tested against a 2D Gaussian null hypothesis H0. With this method, all
known TeV SNR shells falling in the HGPS region (RX J1713.7-3946, Vela Jr., HESS J1731-347, RCW 86) were re-identified with high significance. Second, new significant shell-like structures that emerged from this search were individually re-analysed.
(1) H.E.S.S. (High Energy Stereoscopic System) is a system of Cherenkov telescopes operated in the Khomas Highlands in Namibia. H.E.S.S. phase I started in 2003 with four 12 m telescopes. H.E.S.S. phase II started in 2012 with the commissioning of the additional fifth, 28 m telescope that has been operated together with the other telescopes.
SEARCH FOR NEW SUPERNOVA REMNANT SHELLS IN THE GALACTIC PLANE WITH H.E.S.S. 3
Since the two models are non-nested, the Akaike Information Criterion (AIC) was used to compare them [5]. Figure 1 shows two new shell-like structures that emerged from the grid search. If the source shows a significant shell-like structure in TeV but lacks of a counterpart in lower energy bands, it is classified as TeV SNR candidate. If a strong SNR candidate counterpart at lower energy bands can be associated to the TeV source, it is then identified as new SNR.
3. – Conclusions
As highlighted also in the presented work, the current generation of TeV instruments allows the detection of new SNRs. Studying SNRs emitting at TeV is particularly in-teresting, as the γ-ray emission from these objects might stem from hadronic processes. However, since the emission could also be caused by inverse Compton scattering of elec-trons off low-energy photons, looking for the presence (absence) of a non-thermal X-ray component would help in ruling out one of the two scenarios. For the time being, no X-ray counterpart could be clearly associated to the presented sources; their re-observation at X-ray energies will help to establish the nature of the parent population whose γ-ray emission is observed in the TeV band.
∗ ∗ ∗
The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the French Ministry for Re-search, the CNRS-IN2P3 and the Astroparticle Inter- disciplinary Programme of the CNRS, the UK Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Czech Science Foundation, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and Na-tional Research Foundation, and by the University of Namibia. We appreciate the ex-cellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equip-ment. The shell search performed on the entire HGPS data set employed Gammapy routines as presented in [6].
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[5] Akaike H., IEEE Trans. Automatic Control, 19 (1974) 716. [6] Donath A. et al., PoS(ICRC), 789 (2015).
